The present invention relates to noise reduction systems for television systems in which use is made of integration of video from preceding frames to reduce the noise in the video signal.
The signal content of a video signal is strongly correlated from frame to frame whereas the noise content of of a video signal is uncorrelated. This difference between video signal and noise has been used to reduce noise in a displayed picture. In U.S. Pat. No. 4,064,530, a frame store, capable of storing an entire frame of television video, subtracts digital signals representing corresponding picture elements of stored video and new video to determine the proportion of new video to be mixed with recirculated old video in the frame memory. In a substantially stationary picture (zero motion), a high proportion of recirculated video from the memory is restored therein accompanied by a small proportion of new video. When comparison of corresponding picture elements indicates a substantial difference therebetween (presumably resulting from motion in the scene), a greater proportion of new video and a smaller proportion of recirculated video is stored for that picture element in the frame memory. This system has the disadvantage that the incoming new video is directly mixed with the previously stored video signal and the effective component of the new video may be eliminated therefrom together with the noise component when there is only a small amount of difference between subsequent frames of the video signal. In addition, since the noise component itself never appears as a separate signal, this system requires two sets of circuits for controlling the mixture of new and recirculated video and suffers from the difficulty of operating two such circuits in a completely balanced manner.